1. Field of the Invention
The present invention relates to a neutral position detector for detecting neutral position of a vehicle steering wheel.
2. Description of the Related Art
Neutral position detectors are generally employed in electric control units for vehicle suspensions units or devices which emit alarm signals whenever the steering wheel is turned beyond a predetermined position or angle. A conventional neutral position detector for a steering wheel is illustrated in FIG. 16 as a rotation angle detector for a steering wheel. A rotor 102 is connected to a steering shaft 101 so as to rotate integrally therewith. The rotor 102 has a plurality of peripheral slots 103 for detecting the rotation angle of a steering wheel (not shown). The rotor 102 also has an inner slot 104 for detecting the neutral position of the steering wheel. A detector 105 is disposed adjacent to the rotor 102, and includes a photo-interrupter 106 for detecting the rotation angle, a photo-interrupter 107 for detecting the rotational direction and a photo-interrupter 108 for detecting the neutral position. When the steering wheel is turned 360.degree., the rotor 102 is also turned 360.degree. in synchrony with the steering shaft 101. The photo-interrupter 108 detects the slot 104 every time the rotor 102 is turned 360.degree., and outputs a detect signal. Accordingly, the neutral position detection signal is given plurally corresponding to the number of rotations of the steering wheel. The number of rotations of the steering wheel can be detected by counting the number of the neutral position detection signals by means of an electronic counter.
When the rotor 102 is rotated, the photo-interrupters 106 and 107 detect the rotation angle detection slots 103, and output signals indicative of the rotation angle and the rotation direction of the rotor 102 and of the steering wheel. Thus, the position of the steering wheel can be detected from the number of rotations, the rotation angle and direction of the steering wheel.
A power supply to the neutral position detector is interrupted when the engine is not running in order to optimize consumption. Therefore, a memory device is incorporated into the electronic circuit so that the position of the steering wheel before the engine is stopped may be stored therein.
However, if the steering wheel is turned when the engine is not running, the position of the steering wheel stored in the memory device and the real position thereof will not match. Such mismatch occurs because the position of the steering wheel is detected based on the change in the number and phase of the detection signals from the respective photo-interrupters 106 to 108. Accordingly, the neutral position detector does not detect any change in the number of detection signals when the steering wheel is not rotated, and when the engine is started, although it can detect the slots of the rotor. Therefore, if the steering wheel is turned while the engine is stopped, the detector cannot detect the position of the steering wheel.
Additionally, the detection signals from the respective photo-interrupters 106 to 108 must be processed suitably in the electronic circuit even when only the neutral position of the steering wheel is to be detected. The processing procedures are complex, and thus requires a complicated electronic circuit.